Wireless Sensor Networks (WSN) present a pending challenge for a complete deployability due to energy requirements. The Self-Powered WSN approach aims to extend the sensor node life by means of Energy Harvesting. The harvested energy presents an erratic behavior in both time and frequency. In this paper, a new concept of switch-mode electronically tunable band-pass filters is presented to adaptively follow the power source variations and to maximize the power transfer. To implement these switch-mode filters, three alternatives are presented. These filter topologies are modeled and evaluated. Additionally, some design guidelines are provided. The results show how these high efficiency topologies present a band-pass behavior whose center frequency can be electronically tuned over one decade. The results target the integration of these high-efficiency switch-mode band-pass filters into the future harvesting front-ends.